PERFORMANCE CHARACTERIZATION OF A MULTI-STAGE SOLAR STILL - Energy Research Developments: Tidal Energy, Energy Efficiency and Solar Energy - page 135

This experimental setup is completed by a measurement device: about ten thermocouples
have been placed along the height on each of the internal faces of the active walls. To
measure the flow rates, a weighing machine with a sensitivity of 0.1g and a chronometer have
been used. A typical experience consists in imposing a heat flux density while simultaneously
fixing the flow rate and the temperature of the feed water as well as the temperature of the
condensation wall. When the steady state regime is reached, the temperature of the liquid film
along the heated wall and the distillated flow rate are recorded. The temperature acquisition is
automated by means of an interface card and a micro-computer. It must be noticed that the
thermocouples have been installed at the contact of the wetted face of the heated wall and that
the recorded measurement consequently indicates the temperature of the solid-liquid
interface.

Results

In a previous work [14], it was shown that the behavior of the cell depends on
parameters:

• the temperature T c of the condensation plate .

• the heat flux density q f ,

• the salted water mass flow rate m feed ,

• the temperature of the feed water T feed ,

In practice, the temperature of the condensation wall is imposed by the initial climatic
conditions where the distiller operates and varies in a relatively low domain. This, in the
present work, the influence of the last three parameters on the yield of the distillation cell has
been studied through the use of heat balances obtained from the experimental results.
The heat balance at the level of the liquid film can be formulated as

q = q + q + q + q

(1)

f l s cv p

with q f : heat flux density provided to the liquid film

q l : heat flux density consumed by the phase change
q out: heat flux density evacuated by the salted water
q cv : heat flux density exchanged by convection between the liquid and the gas
q loss : heat flux density lost
The heat flux density transformed into latent heat is expressed as

q l = m v L v (2)

where m v is the evaporated mass flow rate.
The heat flux density evacuated by the brine can be evaluated by using